Most probably because although you may think so, your ears are not exactly alike. The ear is a very complex piece of anatomy and the tiniest difference between your left and right earways and bone structure will change the way different frequencies are channelled down the canal and converted to nerve impulses. Also, any build-up of wax in the ear will make a difference. Another factor is the way your brain responds to the pattern of nerve signals to convert the vibrating air into the concept you call 'sound'. Again, this may be subtly different between the left and right pathways.